1. Technical Field
The present invention relates to optical signal processing, and more particularly to an optical signal processing method and apparatus which produces a predetermined pattern of time delays in a signal.
2. Discussion
The introduction of a predetermined time delay into a signal is an important function in many optical and electrical systems. One technique for providing time delay is known as phase shifting. However, phase shifting cannot be utilized for continuous wave signals with finite bandwidth nor for pulse digital applications; for these types of signals true time delay is necessary.
True time delay is typically generated in an optical signal by a set of optical fibers of different lengths, in conjunction with a switching mechanism to select the fiber of the proper length. However, these types of delay generators have significant problems associated with the large insertion loss of the "one to many" switching mechanism and the large number of fibers necessary for achieving a wide range of optical delays. Further, such delay units require active switching which requires active control signals and drive electronics that add significantly to the overall complexity, bulk and cost of the system. control signals and drive electronics that add significantly to the overall complexity, bulk and cost of the system.
One important application for delay elements is in RF phased-array antennas. In general, antenna beam steering is accomplished by a combination of true time delay and pure phase shifting. Each steering technique may in principle, be implemented either with optical or conventional electronic hardware. Broad band phased-array antennas require true time delayed devices positioned at each antennas subarray to limit the effects of beam squint. Beam squint is the most significant bandwidth limiting effect in phased-array antennas that require wide angle scanning. "Squint," is a phenomena where an antenna points in different directions for different frequencies within the bandwidth of the antenna. Conventional RF phased-array antennas achieve true time delay by using switched lengths of co-axial cable or waveguide. A control system must then calculate, based on the required antenna beam direction, the requisite time delays for each time-delay unit. Control signals are distributed to the subarrays, where the proper length of cable is switched into place. Present optical true time delay approaches mimic the conventional RF approach. Also, conventional phased array RF antennas become very lossy at high RF frequencies. Thus, there is a need for exploiting the intrinsic low loss propagation properties of optical fiber and implementing true time delay using optical approaches.
In one optical implementation, binary-length fiber segments are switched in or out of a given delay path to realize the required time delay. In another implementation, the optical signal is split into fiber paths of all possible desired delay lengths; the signal path of the desired length is enabled and the others are suppressed. Either of these schemes are plagued by significant insertion losses, physical bulk, and the continued reliance on a complex electronic control network. Furthermore due to these disadvantages, the ability to alter the time delay to change beam direction is hindered.
In addition to the phased-array antenna application, similar problems occur in optical processors relying on coherent techniques such as phase quadrature. Phase shifters will impart the proper shift only for a limited bandwidth of frequencies. Other signal processing techniques requiring substantial time delays, such as auto correlation, will also be severely limited in bandwidth if phase-only techniques are utilized. True time delay is also essential in digital applications such as optical clock distribution in large reconfigurable electronic circuitry. Phase is not well defined in a digital pulse train unless it is of constant frequency (f). Phase shifting of such pulse trains will only work for time delays up to 1/f. Phase shifting of pulse trains is usually accomplished by true time delaying the pulse train.
Thus, it would be desirable to provide an apparatus for producing variable true time delay in an optical signal without requiring active switching and without high insertion loss. Accordingly, it would be desirable to provide a technique for producing true time delay in an optical signal which is generally passive in operation. Providing these features in an apparatus which is not limited in RF bandwidth would also be desirable. It would also be desirable to provide a system for easily and rapidly changing a delay pattern in a signal, for example, to facilitate beam steering in a phased-array antenna. In addition, it would be desirable to provide such a true time delay system which is relatively simple, compact and inexpensive.